To simplify the following discussion, consider the output stage of a cellular telephone. The output stage includes a power amplifier that drives an antenna. The efficiency of the power amplifier will be defined to be the power delivered into the load divided by the power consumed by the power amplifier. The efficiency of the amplifier depends on the impedance of the load being driven. For any given power level, a fixed matching network can be included between the antenna and the power amplifier such that the efficiency of the power amplifier is maximized.
Unfortunately, the power amplifier in certain cellular telephones must operate at a number of different power outputs. The power output by the cellular telephone is varied such that the signal received by the base station is more or less constant independent of the distance from the cellular telephone to the base station. When the cellular telephone is far from the base station, the output amplifier is set to generate the maximum allowed power. When the cellular telephone is close to the base station, the power level is reduced to a small fraction of the maximum allowed power.
A fixed matching network can only provide optimum power efficiency at one power level. In cellular communications, the power amplifier is typically set to provide high efficiency at the maximum power level. For example, the power efficiency of a cellular telephone varies from a few percent at the lowest power output setting to approximately 50% at the maximum power setting. This decreased power efficiency decreases the time interval between required recharging, and hence, is undesirable.
In principle, an active matching network can be utilized to provide a more constant power efficiency as a function of the output power of the power amplifier. However, such networks require power and add additional components that increase the cost of the cellular telephone.